The present invention relates to the generation of narrow optical pulses. More specifically, the invention relates to techniques for generating optical pulses at a given repetition rate or frequency f by driving a Mach-Zehnder (M-Z) modulator with a sine wave having a frequency f/2 that is one half of the desired repetition rate f and an amplitude that is twice the amplitude required for switching the M-Z modulator output from minimum to maximum transmission (Vpi).
The basic principle of operation of such a solution, which is widely known in the art, is schematically depicted in FIGS. 1 and 2 of the annexed drawings.
Specifically, in the lower portion of FIG. 1 reference SW designates a modulating sine wave oscillating between xe2x88x92Vpi and +Vpi (another current designation for Vpi being Vxcfx80) while P generally designates the output optical pulse generated by the M-Z modulator.
In FIG. 2, the upper diagram shows the theoretical time behaviour of the pulses expected to be generated by resorting to such a technique. The diagram of FIG. 2b shows corresponding experimental results, which are in good agreement with the theoretical predicted results.
The prior art technique considered here enables pulses to be generated with a nearly fixed duty cycle of about 35%. A typical half-amplitude width value for such pulses is in the range of 33 ps with a repetition period (i.e. the period T=1/f where f is the repetition rate) of 100 ps. Narrower pulses can be obtained by increasing the modulation amplitude, but this gives rise to undesired ringing in pulse tails.
The present invention aims at providing an improved solution enabling narrower pulses be obtained while avoiding the disadvantages encountered when such narrower pulses are attempted to be generated by prior art techniques.
According to the present invention such an object is achieved by means of a method having the features set forth in the annexed claims. The invention also relates to the corresponding system.
By means of the invention, pulse width can be reduced from the typical value of 35% of the repetition period ideally to 0 and in practice to values in the range of about 15% the repetition period. This is particularly advantageous in long haul optical transmission and in optical systems operating at very high transmission rates e.g. in the range of 40 Gbit/s and above.
The invention provides for the pulse width being made selectively adjustable, while totally avoiding pulse ringing.
Essentially, the solution of the invention is based on the concept of adding to the driving signal at frequency f/2 its odd harmonics (i.e. 3/2f, 5/2f, . . . ) with proper amplitudes and phases.